A Brayton Cycle gas turbine engine extracts energy from a flow of gas to produce shaft power, thrust, compressed air, or a combination of these, and generally comprises a turbine, a compressor coupled to the turbine, and a combustor, which receives compressed air or gas from the compressor and in which energy is derived from the combustion of a fuel. Optionally, the system can also include a recuperator, which is a type of heat exchanger that uses the waste heat from the turbine exhaust gas to preheat the compressor discharge air prior to its entering the combustor, thereby increasing the thermodynamic efficiency of the gas cycle.
There is illustrated in FIG. 18 of the drawings a schematic of a recuperated Brayton Cycle gas turbine engine (reference Moran, M. and Shapiro, H., Fundamentals of Engineering Thermodynamics, 2000, Wiley, at pp. 452-456). The principal gas thermodynamic states characterizing the Brayton Cycle are identified in FIG. 18 as compressor inlet 1, compressor discharge 2, combustor exit or turbine inlet 3, and turbine exhaust 4, while the recuperator-specific states are identified as preheated compressor flow 5 and final exhaust 6.
The thermal efficiency N of the recuperated Brayton Cycle turbine engine is given by
  N  =            Wt      -      Wc              Qc      -      Qr      hereinafter referred to as Equation 1, wherein, in terms of gas enthalpy at each of the states as labeled in FIG. 18, and denoted by H, and the gas flow rate M, the turbine work is expressed as Wt=M(H3−H4), the compressor work is Wc=M(H2−H1), the heat supplied by the combustor is Qc=M(H3−H5), and the recuperated heat is Qr=M(H5−H2).
Theoretically, the maximum amount of heat that can be recuperated for compressor discharge flow preheat would occur if all the heat from the turbine exhaust could be transferred to the compressor flow, and is given by Qr max=M(H4−H2). In practice, the recuperated heat is only a fraction of the maximum, and is set by the geometric and flow characteristics of the heat exchanger used as recuperator. This fraction is defined to be the recuperator heat transfer effectiveness (Incropera, F. P. and DeWitt, D. P., Fundamentals of Heat and Gas Transfer, Wiley, 1996, at p. 600), denoted by Er, and expressed as Er=Qr/Qr max, or
  Er  =                    H        ⁢                                  ⁢        5            -              H        ⁢                                  ⁢        2                            H        ⁢                                  ⁢        4            -              H        ⁢                                  ⁢        2            hereinafter referred to as Equation 2.
Combining both equations 1 and 2, one obtains the correlation between cycle efficiency and recuperator effectiveness:
  N  =                    (                              H            ⁢                                                  ⁢            3                    -                      H            ⁢                                                  ⁢            4                          )            -              (                              H            ⁢                                                  ⁢            2                    -                      H            ⁢                                                  ⁢            1                          )                            (                              H            ⁢                                                  ⁢            3                    -                      H            ⁢                                                  ⁢            2                          )            -                        (                                    H              ⁢                                                          ⁢              4                        -                          H              ⁢                                                          ⁢              2                                )                ⁢        Er            hereinafter referred to as Equation 3.
In an ideal situation, a recuperator with Er approaching 1 (or 100% effectiveness) would raise the enthalpy (or more tangibly, the temperature) of the compressor discharge isobarically to match the enthalpy of the turbine exhaust with no pressure drop to burden the engine, thereby attaining the recuperated engine's maximum theoretical thermal efficiency, given by Nmax=1−(H2−H1)/(H3−H4).
In practical terms, a heat exchanger cannot achieve 100% effectiveness or zero pressure drop impact on the engine. However, in order to satisfy the requirements of a gas turbine operating within a given range of conditions, a recuperator should desirably be designed to maximize heat transfer effectiveness while minimizing the pressure impact or penalty on engine operation. These two conditions have typically involved competing design requirements, which result in difficult tradeoffs that must be balanced according to the specific needs of the target application.
A heat exchanger in accordance with the present invention is provided which allows design freedom to manipulate particularly the cross-section (but also the flow paths there through) of the flow passages to maximize heat transfer and minimize pressure loss, as considered best for the particular application of interest. As can be seen from equation 3, in addition to effectiveness, each of the enthalpies h has an influence on the thermal efficiency.
While the present invention should not be considered as being limited to any particular size of gas turbine or other engine, it is considered very suitable for small gas turbines, i.e., those in the meso-scale and micro-scale range. Meso-scale and micro-scale engines are defined for the purposes of this specification and the claims as engines having power outputs of 100 watts to 15 kW (kilowatts) and 15 to 200 kW respectively.
It is an object of the present invention to provide a meso-scale gas turbine engine (as well as a micro-scale gas turbine engine) or other ultra-small size engine, for example, a gas turbine providing portable power for a foot soldier. Such a meso-scale engine may have a size of about 8 kW equaling about 10 hp (horsepower) and may weigh about 10 pounds plus the weight of any recuperator.
In order to be considered suitable for addition of a recuperator so that it will work properly, it is considered that a gas turbine should have a low pressure ratio (the ratio of the high inlet pressure to the gas turbine to the low outlet pressure thereof) on the order of 5:1 or less, for conditions typical of a meso-scale or micro-scale gas turbine with normal component adiabatic efficiencies. If the adiabatic efficiency of the turbine and compressor are high enough, which however is normally difficult to achieve, the pressure ratio where a recuperator could be used would be higher. Inspection of a Brayton Cycle T-s (temperature-entropy) state diagram shows that this pressure ratio makes them well suited to derive benefits from the use of heat recovery devices like recuperators (El Wakil, M., Powerplant Technology, McGraw-Hill, 1984, at pp. 323-324), since the turbine exhaust temperature can be significantly higher than the compressor discharge temperature. While larger gas turbines may often have higher pressure ratios and thus be considered unsuitable for the addition of recuperators, meso-scale and micro-scale gas turbine engines are typically characterized by operating at low pressure ratios, i.e. in the range of 2 to 3, thus making them suitable for the addition of recuperators for increasing efficiency. Non-recuperated meso-scale and micro-scale gas turbine engines typically present thermal or cycle efficiencies in the range of 7% to 13%, with engine thermal efficiency defined as the ratio between net work produced and heat input. It is considered desirable to substantially increase this efficiency, by the addition of a suitable recuperator.
In their efforts to develop such a gas turbine engine in which a suitable higher efficiency could be achieved, the inventors of the present invention received bids for recuperators which would undesirably weigh in excess of 100 pounds. They were not able to find commercially a recuperator providing the desired high efficiency but of a suitably light weight so that the gas turbine engine could still be light enough so that the recuperated gas turbine could be portable by a foot soldier.
For the operating conditions typical of a meso-scale or micro-scale gas turbine, as will become more apparent hereinafter, a tripling of the thermal efficiency by use of such a heat exchanger would not be unexpected.